Motor vehicle suspension systems are configured so that the wheels are able to follow elevational changes in the road surface as the vehicle travels therealong. When a rise in the road surface is encountered, the suspension responds in “jounce” in which the wheel is able to move upwardly relative to the frame of the vehicle. On the other hand, when a dip in the road surface is encountered, the suspension responds in “rebound” in which the wheel is able to move downwardly relative to the frame of the vehicle. In either jounce or rebound, a spring (i.e., coil, leaf, torsion, etc.) is incorporated at the wheel in order to provide a resilient response to the respective vertical movements with regard to the vehicle frame. However, in order to prevent wheel bouncing and excessive vehicle body motion, a damper (i.e., a shock absorber, strut, etc.) is placed at the wheel to dampen wheel bounce. Additionally, when the limit of jounce is encountered, it is customary to provide a maximum jounce impact absorber in the form of a bumper cushion.
Referring now to FIGS. 1A through 1C, components of a conventional suspension system 10 are depicted which allow for jounce and rebound at a wheel of the subject motor vehicle 12.
Firstly with regard to FIG. 1A, a control arm 14 is pivotally mounted with respect to the frame 16, wherein, in the depicted example, a torsion spring 18 is utilized to provide resilient response for the jounce and rebound of the control arm relative to the frame. To provide control over the rate of jounce and rebound, a damper 20 in the form of a shock absorber is connected pivotally at one end to the frame 16 and connected pivotally at the other end to the control arm 14. Alternatively, a damper in the form of a strut (i.e., a shock absorber and spring combination) may be used in the suspension system, as for example disclosed in U.S. Pat. No. 5,467,971. To provide cushioning in the event a maximum jounce occurs, a jounce bumper cushion 22 is mounted to the frame 16 which is resiliently compressed by movement of the control arm as jounce approaches its maximum.
Referring next to FIG. 1B, the internal components and operational aspects of a conventional damper 20 are schematically depicted. A valved piston 30 is reciprocably movable within a damper cylinder 32. A damper rod 34 is attached to the valved piston 30 and is guided by a damper rod guide 36 at one end of the damper cylinder 32. Below the valved piston 30 and above the damper rod guide 36 is a mutually interacting rebound limiter 38. The instantaneous position of the valved piston 30 within the damper cylinder 32 defines a first interior portion 32F and a second interior portion 32S of the interior of the damper cylinder. In the example depicted at FIG. 1B, the pressurization in the first and second interior portions 32F, 32S is provided by an oil O which is pressurized by pressurized gas, preferably nitrogen. The oil is able to directionally meter through valving 40 of the valved piston 30 in a manner which provides damping. The rod guide 36 carries a rod seal 42 which are covered by an end cap 44.
As can be understood from FIG. 1C, during operation of a damper, the damper rod 34′ axially moves with respect to the rod seal 42′. Over time, the rod seal 42′ may become damaged, the damper rod 34′ may become scratched, or the dimensional fit between the rod seal and the damper rod may have untowardly changed (as for example due to an excessive side-load) such that oil from within the damper cylinder 32′ exteriorly passes between the rod seal and the damper rod in the form of leaked oil OL′. The leaked oil OL′ will pass out of the end cap 44′ and become visible as drips, etc., whereupon the damper is interpreted as being “bad”, requiring replacement. However, in actuality, the mere presence of leaked oil does not necessarily mean the damper needs replacement (i.e., that it is “bad”), in that there may yet remain a fully serviceable amount of oil thereinside. By way of example, it may be possible that between about 3 and 10 percent of the oil volume could leak before the operation of the damper would be compromised.
Thus, there remains in the damper art the problem of determining when leaked oil represents a true problem indicative of the need to replace the damper.